When compared with the parameters that indicate the pozzolanic activity, the compressive strength of the geopolymeric paste prepared with dacitetuff calcined at700°C attains thehighest value equal to 42.5 MPa at the usual curing temperature of 40°C (Figs. 5 and 6). This is thought to be the result of activation of the zeolite, which converts from clinoptilolite to mordenite at 700°C. Continued heat treatment up to 800°C converts mordenite to opal, which reacts strongly with alkaline solutions and gives a compressive